It started annoying me that for example ReaFIR wouldn’t trigger the offline processing when the frequency response was manipulated, because no callback from the plugin happens when doing that. With some win32-based hackery I was able to make it work. (ReaFIR is specially hardcoded in HourGlass to work like this now, the hack needs to be refined to handle more of the standalone ReaPlugs.) (click image for animation)

It probably took a bit more effort and nasty looking code than should have been dedicated to doing this, but I think it’s a pretty neat hack anyway. And I (re)learned some useful tricks. (Thanks to Justin Frankel for the help on this.)